In the production of vinyl halide polymers, it is common practice to stop the polymerization reaction by which these polymers are produced at a stage prior to that at which the reaction would ordinarily come to a halt in order to obtain stable uniform products having the desired porosity, melt viscosity, and other properties. Chain termination is usually effected by incorporating in the polymerization mixture a compound that will act as chain-terminating or shortstopping agent for the polymerization reaction without adversely affecting the color of the vinyl halide polymer. The chain-terminating agent also serves to prevent polymerization of recovered monomer and to prevent degradation of the polymer when it is heated during the monomer-removal steps and/or the drying step of the process.
In U.S. Pat. No. 2,662,867, Hoertz disclosed the use of water-soluble phenols, quinones, and nitro-aromatic compounds as chain-terminating agents in the preparation of vinyl chloride polymers. He taught that these compounds function as free radical scavengers that react with the free radicals generated by residual catalyst, from active polymer chains, and from reaction between unreacted monomer and residual. He further indicated that the addition of these chain-terminating agents stabilizes the polymer not by reacting with the liberated hydrohalogen acid but rather by destroying the free radicals in the dispersion before polymer decomposition is inaugurated.
In the commercial production of vinyl chloride polymers, one of the most frequently used chain-terminating agents is 2,6-di-tertiary butyl-p-cresol (BHT). 2,6-Di-tertiary butyl-p-cresol not only acts as an efficient chain-terminating agent during the final processing steps and yields polymers having the desired combination of physical properties but also provides some improvement in the long term heat stability of the finished resin.